105 research outputs found
The Theory of Scanning Quantum Dot Microscopy
Electrostatic forces are among the most common interactions in nature and
omnipresent at the nanoscale. Scanning probe methods represent a formidable
approach to study these interactions locally. The lateral resolution of such
images is, however, often limited as they are based on measuring the force
(gradient) due to the entire tip interacting with the entire surface. Recently,
we developed scanning quantum dot microscopy (SQDM), a new technique for the
imaging and quantification of surface potentials which is based on the gating
of a nanometer-size tip-attached quantum dot by the local surface potential and
the detection of charge state changes via non-contact atomic force microscopy.
Here, we present a rigorous formalism in the framework of which SQDM can be
understood and interpreted quantitatively. In particular, we present a general
theory of SQDM based on the classical boundary value problem of electrostatics,
which is applicable to the full range of sample properties (conductive vs
insulating, nanostructured vs homogeneously covered). We elaborate the general
theory into a formalism suited for the quantitative analysis of images of
nanostructured but predominantly flat and conductive samples
The origin of high-resolution IETS-STM images of organic molecules with functionalized tips
Recently, the family of high-resolution scanning probe imaging techniques
using decorated tips has been complimented by a method based on inelastic
electron tunneling spectroscopy (IETS). The new technique resolves the inner
structure of organic molecules by mapping the vibrational energy of a single
carbonmonoxide (CO) molecule positioned at the apex of a scanning tunnelling
microscope (STM) tip. Here, we explain high-resolution IETS imaging by
extending the model developed earlier for STM and atomic force microscopy (AFM)
imaging with decorated tips. In particular, we show that the tip decorated with
CO acts as a nanoscale sensor that changes the energy of the CO frustrated
translation in response to the change of the local curvature of the surface
potential. In addition, we show that high resolution AFM, STM and IETS-STM
images can deliver information about intramolecular charge transfer for
molecules deposited on a~surface. To demonstrate this, we extended our
numerical model by taking into the account the electrostatic force acting
between the decorated tip and surface Hartree potential.Comment: 5 pages, 4 figure
The mechanism of high-resolution STM/AFM imaging with functionalized tips
High resolution Atomic Force Microscopy (AFM) and Scanning Tunnelling
Microscopy (STM) imaging with functionalized tips is well established, but a
detailed understanding of the imaging mechanism is still missing. We present a
numerical STM/AFM model, which takes into account the relaxation of the probe
due to the tip-sample interaction. We demonstrate that the model is able to
reproduce very well not only the experimental intra- and intermolecular
contrasts, but also their evolution upon tip approach. At close distances, the
simulations unveil a significant probe particle relaxation towards local minima
of the interaction potential. This effect is responsible for the sharp
sub-molecular resolution observed in AFM/STM experiments. In addition, we
demonstrate that sharp apparent intermolecular bonds should not be interpreted
as true hydrogen bonds, in the sense of representing areas of increased
electron density. Instead they represent the ridge between two minima of the
potential energy landscape due to neighbouring atoms
Electron spin secluded inside a bottom-up assembled standing metal-molecule nanostructure
Artificial nanostructures, fabricated by placing building blocks such as
atoms or molecules in well-defined positions, are a powerful platform in which
quantum effects can be studied and exploited on the atomic scale. Here, we
report a strategy to significantly reduce the electron-electron coupling
between a large planar aromatic molecule and the underlying metallic substrate.
To this end, we use the manipulation capabilities of a scanning tunneling
microscope (STM) and lift the molecule into a metastable upright geometry on a
pedestal of two metal atoms. Measurements at millikelvin temperatures and
magnetic fields reveal that the bottom-up assembled standing metal-molecule
nanostructure has an spin which is screened by the substrate
electrons, resulting in a Kondo temperature of only mK. We extract
the Land\'e -factor of the molecule and the exchange coupling to the
substrate by modeling the differential conductance spectra using a third-order
perturbation theory in the weak coupling and high-field regimes. Furthermore,
we show that the interaction between the STM tip and the molecule can tune the
exchange coupling to the substrate, which suggests that the bond between the
standing metal-molecule nanostructure and the surface is mechanically soft
Scanning Quantum Dot Microscopy
Interactions between atomic and molecular objects are to a large extent
defined by the nanoscale electrostatic potentials which these objects produce.
We introduce a scanning probe technique that enables three-dimensional imaging
of local electrostatic potential fields with sub-nanometer resolution.
Registering single electron charging events of a molecular quantum dot attached
to the tip of a (qPlus tuning fork) atomic force microscope operated at 5 K, we
quantitatively measure the quadrupole field of a single molecule and the dipole
field of a single metal adatom, both adsorbed on a clean metal surface. Because
of its high sensitivity, the technique can record electrostatic potentials at
large distances from their sources, which above all will help to image complex
samples with increased surface roughness.Comment: main text: 5 pages, 4 figures, supplementary information file: 4
pages, 2 figure
Electron energy loss spectroscopy with parallel readout of energy and momentum
We introduce a high energy resolution electron source that matches the
requirements for parallel readout of energy and momentum of modern
hemispherical electron energy analyzers. The system is designed as an add-on
device to typical photoemission chambers. Due to the multiplex gain, a complete
phonon dispersion of a Cu(111) surface was measured in seven minutes with 4 meV
energy resolution
kMap.py: A Python program for simulation and data analysis in photoemission tomography
For organic molecules adsorbed as well-oriented ultra-thin films on metallic
surfaces, angle-resolved photoemission spectroscopy has evolved into a
technique called photoemission tomography (PT). By approximating the final
state of the photoemitted electron as a free electron, PT uses the angular
dependence of the photocurrent, a so-called momentum map or k-map, and
interprets it as the Fourier transform of the initial state's molecular
orbital, thereby gains insights into the geometric and electronic structure of
organic/metal interfaces.
In this contribution, we present kMap.py which is a Python program that
enables the user, via a PyQt-based graphical user interface, to simulate
photoemission momentum maps of molecular orbitals and to perform a one-to-one
comparison between simulation and experiment. Based on the plane wave
approximation for the final state, simulated momentum maps are computed
numerically from a fast Fourier transform of real space molecular orbital
distributions, which are used as program input and taken from density
functional calculations. The program allows the user to vary a number of
simulation parameters such as the final state kinetic energy, the molecular
orientation or the polarization state of the incident light field. Moreover,
also experimental photoemission data can be loaded into the program enabling a
direct visual comparison as well as an automatic optimization procedure to
determine structural parameters of the molecules or weights of molecular
orbitals contributions. With an increasing number of experimental groups
employing photoemission tomography to study adsorbate layers, we expect kMap.py
to serve as an ideal analysis software to further extend the applicability of
PT
How cold is the junction of a millikelvin scanning tunnelling microscope?
We employ a scanning tunnelling microscope (STM) cooled to millikelvin
temperatures by an adiabatic demagnetization refrigerator (ADR) to perform
scanning tunnelling spectroscopy (STS) on an atomically clean surface of
Al(100) in a superconducting state using normal-metal and superconducting STM
tips. Varying the ADR temperatures between 30 mK and 1.2 K, we show that the
temperature of the STM junction is decoupled from the temperature of the
surrounding environment . Simulating the STS data with the
theory, we determine that K, while the
fitting of the superconducting gap spectrum yields the lowest mK.Comment: 12 pages, 10 figure
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